Are there any issues with a custom subdomain transition taking longer than TTL?

We have been trying unsuccessfully to set the blog to use a subdomain. It is currently set up using a custom domain, but we wish to change it to a subdomain. This subdomain option has been purchased additionally, and the CNAME record is in place against the blog name ngrainblog.wordpress.com (per the documentation). The CNAME has a short TTL, I found that immediately after the setting is made, the subdomain works to access the site.. but after some time, it stops working... so we have had to flip it on, and then revert a number of times, so this is quite frustrating. With a short TTL (5 minutes) there should not be much downtime during the transition, however when it stops working, it is not able to find the site at all.

Just to check whether I understand this correctly, you have a blog which you want to map as a subdomain, and it works for a bit but then stops. You are trying to switch from:

ngrainblog.wordpress.com mapped to ngrainblog.com

to

ngrainblog.wordpress.com mapped to subdomain.ngrainblog.com and ngrainblog.com mapped to something else.

Is this correct? If so it occurs to me that the mapping of ngrainblog.com to something else means mapping the DNS servers bac to your hosted domain (unless ngrainblog.com points to another WordPress.com blog). This can take up to 72 hours to take affect.

Also, the CNAME for the subdomain will have to be added on your original host's DNS servers, not the WordPRess.com ones.

At a pure guess, could you be setting the CNAME on the WordPress.com DNS? If you are then this will stop working once the name server propagation maps the name server back to your host's DNS.